Blender 2.8 - CTRL+R not working(loop cut and slide tool)
Your model is just one big N-GON, and the tool only works on quads. Check out this link to learn a little more about modeling with tris, quads and n-gons. 



This is your model (remade by me) with much better topology.  The loop cut and slide tool will work correctly with this kind of topology.  Also, getting your model to look like this will give you some good modeling practice.